Microstructurings comprise a plurality of dots and/or lines that are used to alter an optical property of a surface or of one or more layers of a storage medium. A change in reflectivity, transmission, absorption, scattering behavior, phase of the reflected light or a combination of all the effects can be achieved through microstructurings. In this case, the spatial resolution may be less than 10 μm down to dot or line dimensions of less than 1 μm. Microstructurings of this type are used for storing information; and in particular, computer-generated holograms, microimages or microscripts can be produced thereby.
Computer-generated holograms comprise one or more layers of dot matrices or dot distributions which, in the event of illumination with a preferably coherent light beam, lead to a reconstruction of items of information coded in the hologram. In this case, the dot distribution may be calculated as an amplitude hologram, phase hologram or as a kinoform Fresnell or Fourier hologram. In order to produce computer-generated holograms, the holograms are first calculated and subsequently written to a storage medium by a suitable writing device by means of dotwise introduction of energy. The resolution of the dot matrix that arises may lie within the range down to less than 1 μm, as already discussed. Consequently, holograms having a high resolution can be written in a confined space, the information of which holograms can only be read out by illumination with a light beam and reconstruction of a diffraction pattern. In this case, the size of the holograms may be between a few square millimeters and multiple square centimeters.
A major advantage of the computer-generated holograms is that each hologram can be calculated individually without a high outlay. Consequently, holograms comprising consecutive numbers or production parameters, for example, can be generated in series. Holograms of this type can therefore be used in particular as security features or in logistics for product tracking on packaging, credit cards, entrance tickets or the like. By a suitable read-out device, the security features of the hologram can be read out and the authenticity and individuality of the security feature can be checked in a simple manner.
The computer-generated holograms described above can be combined with a directly visible item of information (i.e. microscripts and microimages). In addition, with a microstructuring mentioned in the introduction, the above-mentioned microimages and microscripts themselves can also be written independently of computer-generated holograms. The dot distributions can also be produced as dot matrix holograms, wherein in each case individual small area portions are produced as different diffraction structures of the dot matrix hologram. In addition, it is also possible to produce a diffractive optical element (DOE) per se with the microstructuring.
When writing or read-out by a light beam as described below, a laser beam in the visible wavelength range is generally preferred. Nevertheless, the present invention is not restricted to the application of visible light. In principle, the invention can be applied with electromagnetic radiation in a wide wavelength range.
The prior art furthermore discloses a plurality of writing devices for writing computer-generated holograms which write the optical structures of the holograms in planar storage media. By way of example, reference is made in this respect to documents WO 02/079881, WO 02/079883, WO 02/084404, WO 02/084405 and WO 03/012549. These writing devices use a laser beam that successively scans each individual dot of the dot matrix and each may or may not introduce light energy into the storage medium.
A plurality of reading devices are likewise known which are suitable, by illuminating the hologram area by a light beam and a suitable optical unit, for making the reconstruction visible or electronically representable by recording means and evaluable. By way of example, in this context, reference is made to documents DE 101 37 832, WO 02/084588 and WO 2005/111913.
By contrast, EP 1 094 352 A2 discloses an optical writing device for image generation with a light source comprising a plurality of laser diodes arranged in a series. The light emitted by said light source is directed, by an optical unit, onto a grating light valve (GLV) from Silicon Licht Machines, by which a diffraction takes place for each point of the GLV. The GLV can also be referred to as a line light modulator. An image arises line by line as a result of progressive exposure. The resolution in the written image that is generated by the writing device is specified as 2400 dpi corresponding to a dot size of about 10 μm.
DE 198 02 712 A1 discloses a device for exposure of a computer-generated hologram on a storage medium. A conditioned laser beam impinges on a digital light processor (DLP), by which a 2D light field is deflected onto the storage medium. Structures are produced in a storage medium by small-area individual mirrors of the DLP. The hologram size is therefore determined by the DLP used and a scaling of the imaging.
EP 1 202 550 A1 shows a writing device comprising a line light modulator (GLV) and comprising an imaging optical unit. A polarizer is arranged in-a beam path of a laser beam, said polarizer producing a linear polarization of the beam. The preferred direction of the line light modulator is utilized as a result, such that high intensities are achieved on a writing medium. A writing of photosensitive and heat-sensitive materials is thus accelerated.
EP 1 480 441 A1 shows a construction of a writing device comprising a beam multiplier that generates a multiplicity of individual beams from a single-mode laser beam. Said individual beams then impinge on a multichannel spatial light modulator and are individually modulated there. The individual beams reflected by the SLM are each substantially single-mode beams and a downstream optical unit images this plurality of individual beams onto a surface of a light-sensitive workpiece. The method described serves for example to produce circuits.
WO 01/79935 discloses a device which can be used to produce photomasks for semiconductor elements and display devices. Semiconductor elements, displays, integrated optical elements and electronic connecting structures can likewise be written directly. In order to produce individual dots, a spatial light modulator (SLM) is in a beam path of a laser beam. A downstream imaging optical unit then images a light modulator on a storage medium in order to write a structure. In order to control an energy fluctuation between individual writing pulses and thus to enable the structure to be written uniformly, it is proposed to use a very fast control of a switch that enables the light energy to be switched off actually during a laser pulse. In this case, WO 01/79935 takes an average writing power of 10-100 mW as a basis.
The systems described above each has at least one of the following problems, such as an excessively low spatial resolution and low accuracy of the written structuring, an excessively low throughput or writing speed, no individually producible structures or an excessively low writing energy to allow the structuring of materials that are not very photosensitive or heat-sensitive.
Systems of the type described above require for operation in progress an active autofocusing in order to correct relative deviations between the imaging optical unit and the storage medium. In this case, one problem for a very accurate and quickly operating autofocus system is that the structuring of a storage material with a resolution in the range of the wavelength or with a corresponding numerical aperture(NA) necessitates an active height regulation. This results from the limited depth of field, which is related to the NA used as follows:
      Δ    ⁢                  ⁢    z    =      0.8    ⁢                                                            ⁢          λ                          N          ⁢                                          ⁢                      A            2                              .      
It can be seen here that as NA increases, which means smaller structures, the depth of field Δz decreases reciprocally to the square of NA. For structures in the visible wavelength range and given structure sizes of about 1 μm, a depth of field in the single-digit micrometer range is thus calculated. On account of manufacturing tolerances of the storage medium or on account of inaccuracies as a result of the mounting of the storage medium, a height variation of the storage material in the two-digital micrometer range should be expected. This necessitates an active height regulation.
The requirement made of the height regulation consists in keeping the writing beam within the depth of field over its entire region. In the case of systems known from the prior art it was assumed that the height changes are little within the exposure width or region and a one-point height regulation is thus sufficient. However, experiments have repeatedly shown that unstructured locations, particularly at the edge of the exposure, occur on account of inaccuracies in manufacturing and as a result of misalignment of the sample or storage material mounting. This results in the requirement for a height regulation over the entire exposure region which becomes all the more urgent, the larger the exposure region becomes.